1. Field of the Invention
The present invention relates an angular velocity sensor (an angular rate sensor) for detecting a Coriolis force of a vibration mass generated in a direction perpendicular to a driving direction and detecting an angular velocity of rotating motion of a vibration mass if the rotating motion is applied to the vibration mass.
2. Description of the Prior Art
As an angular velocity sensor using semiconductor manufacturing technology, a technology disclosed in Patent application Publication (KOKAI) No. 5-312576 has been known. In this angular velocity sensor, an oxide film is formed on a first silicon substrate, then the oxide film is patterned, then a second silicon substrate is bonded to the first silicon substrate via the oxide film with the use of a silicon direct bonding (SDB) method. Then the second silicon substrate in the SDB structure is polished up to a predetermined thickness, and then the second silicon substrate is etched to form grooves having vertical side walls. The vibration mass, first and second supporting portions, and frame portions of the angular velocity sensor are patterned by using the grooves. Almost whole peripheral side surfaces of the vibration mass are isolated by the side walls of the grooves, but parts of the vibration mass are supported by the first supporting portions. The first supporting portions (on the opposite side to the connection portions to the vibration mass) are connected to the frame portion which is separated from the vibration mass by the grooves. The frame portion is supported by the second supporting portions. Comb electrodes, or the interdigital capacities for driving the vibration mass by an electrostatic force in the x axis direction to vibrate are constituted at a predetermined position of the frame portion. In order to detect displacement of the vibration mass due to the Coriolis force, piezo resistance bridges are formed around the connection portions between the first supporting portions and the frame portion. In addition, detection electrodes are formed in the grooves between the vibration mass and the frame portion. Thus the angular velocity sensor in Patent application Publication (KOKAI) No. 5-312576 (referred to as a "first conventional example") is constituted. Since the vibration mass is formed of a single crystal semiconductor thin film, mass m of the vibration mass is small. Therefore, the Coriolis force generated is slight, and it cannot be measured actually by the resistance bridge consisting of the piezo resistances. For this reason, displacement of the vibration mass must be measured according to change in electrostatic capacitance of a capacitor, i.e., a pair of detection electrodes between the vibration mass and the frame portion. However, because the detection electrodes are formed on surfaces of side walls of the vibration mass and the frame portion, opposing areas of the simple parallel plate capacitor are small so that electrostatic capacitance values cannot be assured. As a result, there has been caused a problem that an S/N ratio cannot be sufficiently assured in measuring displacement of the vibration mass. In addition, there has been another problem that it is difficult to assure sufficient opposing areas in the comb electrodes to drive the vibration mass, so that sufficient vibration amplitude cannot be achieved.
In the first conventional example, there has been still another problem that miniaturization of the supporting portion and the drive electrodes become difficult since electrical connections between piezo resistances or detection electrodes and the comb electrodes via the supporting portions are needed and also electrical isolation therebetween are needed.
In the first conventional example, there has been yet still another problem that, if the electric connection is made by metal such as aluminum, since difference in thermal expansion coefficient is large between the metal connection and the supporting portions such as a silicon and an insulating film, it is likely to cause an offset in the output of the angular velocity sensor because of bowing of the structural body generated by thermal stress. And also it is likely to cause plastic deformation of the metal portion because of significant deformation of the supporting portions. Hence, deterioration of the output of the angular velocity sensor will be caused with elapsed time.
In the meanwhile, as another conventional example, there has been known an angular velocity sensor which is disclosed in, for example, J. Bernstein et al. "Micromachined Comb-Drive Tuning Fork Rate Gyroscope" Digest IEEE/ASME Micro Electro Mechanical Systems (MEMS) Workshop, Florida, 1993, 143-148 (referred to as a "second conventional example"). In the second conventional example, all the structural bodies are formed of polysilicon thin film stacked on a silicon substrate. In other words, the vibration mass, the supporting portions for supporting the vibration mass on the silicon substrate, the comb electrodes, and the drive electrodes constituted by the comb electrodes fixed to the silicon substrate are patterned to be separated by grooves formed in the polysilicon thin film. The angular velocity sensor is so formed that the oxide film is formed on the silicon substrate and the vibration mass is disposed on the oxide film. The vibration mass is separated from the oxide film and the detection electrodes are formed on the oxide film and directly below the vibration mass.
In the angular velocity sensor of the second conventional example, detection of the angular velocity is carried out in such a manner that vibration due to the Coriolis force generated along the detection axis direction is detected in a vibration state of the vibration mass which is driven along the drive axis direction. Accordingly, abnormality to disturb the operation of the vibration mass can be immediately detected by monitoring vibration amplitude along the drive axis direction. However, in the event that the vibration mass can vibrate at a predetermined amplitude along the drive axis direction whereas vibration of the vibration mass exceeding a predetermined amplitude is prevented along the detection axis direction due to enter of movable dust between the vibration mass and the substrate, it is difficult to detect an abnormal state easily by the conventional vibration gyroscope. Further, even if the movable dust disturbing the vibration of the vibration mass exists very near the vibration ranges of the vibration mass along other two orthogonal axis perpendicular to the detection axis, there exists the possibility that a failure of the angular velocity sensor is caused.